Cationic RhI Complexes of Azulenes and Their Catalytic Activity on the Formation of Heptalene‐1,2‐dicarboxylates from Dimethyl Acetylenedicarboxylate and Azulenes

Abstract [Rh1(η5‐azulene)(cod)]+BF complexes 3a–g (cod = (Z,Z)‐cycloocta‐1,5‐diene) have been synthesized by reaction of [Rh1(cod)]+BF in THF with the corresponding azulenes 1a–g (Table 1). The structure of [Rh1(cod)(η5‐guaiazulene)]+BF (3a) has been determined by X‐ray diffraction analysis (Fig. 1 and 2). The Rh‐atom is oriented above the five‐membered ring of the azulene with almost equal RhC distances to all five C‐atoms of the ring. The (Z,Z)‐cycloocta‐1,5‐diene ring occurs in two enantiomorphic distorted (C2v → C2) tub conformations in the crystals (Fig. 3). In CDCl3 solution, the cod ligand in the complexes 3 shows a dynamic behavior on the 1H‐NMR time scale which is best explained by rotation of the cod ligand relative to the azulene ligands around an imaginary codRhazulene axis. The new complexes 3 catalyze the formation of heptalene‐1,2‐dicarboxylates 2 from dimethyl acetylenedicarboxylate (ADM) and the corresponding azulenes 1 just as effectively as [RuH2(PPh3)4] and the analogous [RhH(PPh3)4] complex in MeCN solution (Table 3). On grounds of simplicity, 3 can be generated in situ, when [RhCl(cod)]2 is applied as catalyst (Table 3).